The present invention relates to video quality of service, and more particularly to a method of measuring peak signal to noise ratio (PSNR) of full color video, both total and component contributions.
It is desired to quantify the error in color related components—chrominance, color difference, etc.—of video signals in an objective manner using units compatible with existing industry standards. PSNR is such a method for luminance only, as defined in ANSI® T1.801.03-1996: Digital Transport of One-Way Video Signals—Parameters for Objective Performance Assessment, page 35. An implementation of this standard is found in the PQA200 and PQA300 Picture Quality Analyzers manufactured by Tektronix, Inc., Beaverton, Oreg. PSNR is a measurement that gives one value for each frame or sequence of frames of the video signal under test, given the reference frame or sequence. It is also desirable to have an error map, similar to the luminance difference map, labeled “PSNR” map in the TEKTRONIX® PQA200/300 products, that displays luminance and chrominance errors simultaneously in an intuitive manner. And finally it is desirable to measure the contribution of constituent video components, i.e., Cb of YCbCr, to the total PSNR measurement.
PSNR stands for Peak Signal to Noise Ratio and is proportional to the log of a ratio of root mean square (rms) values. From ANSI® T1.801.03-1996:PSNR(Vpeak,Cin,Cout,Ncol,Nrow,i,j,t,d):=20*log [Vpeak/{SQRT((1/(Ncol*Nrow))*ΣjΣi(Cini,j,t-d−Couti,j,t)2)}where Cin and Cout are general image parameters, Vpeak is the reference peak unit value for Cin and Cout, Ncol is the number of columns (width) of parameter matrix C, Nrow is the number of rows (height) of parameter matrix C, i is the row index, j is the column index, t is the time index, and d is the delay between Cin and Cout. The ratio taken is the peak rms value possible for the measure divided by the actual rms difference between the test (Cout) and reference (Cin) video signals. PSNR for the TEKTRONIX® PQA200/300 products and most measurements within the ANSI® document cited above, as well as other picture quality of service (PQOS) methods, use Cin and Cout taken directly or indirectly from the pixel luminance (Y) values alone. Vpeak is taken as peak white.LumPSNR(Ypeak,Yin,Yout,Ncol,Nrow,i,j,t,d):=20*log [Ypeak/{SQRT((1/(Ncol*Nrow))*ΣjΣi(Yini,j,t-d−Youti,j,t)2)}Thus chrominance errors that occur independent from luminance errors are undetected by these PSNR measures.